Pulp Fibers

With respect to screening, pulp fibers are characterized by a number of physical

properties such as fiber length, fiber flexibility, freeness and disruptive shear

stress of the fiber network. Together with the consistency, these properties determine

the performance of the furnish in a pressure screen.

The influence of fiber flexibility on passage ratio is secondary to the influence of

fiber length. Flexibility plays no role as long as the fibers are shorter than the

width of the slot or the diameter of the hole. As the fibers become longer, however,

the flexible fibers’ passage ratios are higher than those of stiff fibers [2,18]. Note

that fiber stiffness is a function of the temperature, with fibers becoming more

flexible as the temperature rises. Figure 6.16 exemplifies the fiber passage ratio as

a function of the fiber length and hole size.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Fiber passage ratio

Fiber length [mm]

1.00 mm holes

1.75 mm holes

Fig. 6.16 Example of fiber passage ratio as a function of the

fiber length and hole size;smooth hole screen, bump rotor,

softwood thermomechanical pulp (TMP) [8].

Regarding the pulpwood raw material, a distinct difference can be observed between

the long softwood and the short hardwood fibers. The capacity of a given

slotted pressure screen with hardwood pulp is 20–30% higher than its capacity

with softwood pulp.

6.4 Centrifugal Cleaning Theory